Author Topic: Help, 2 & 3 won't fire  (Read 1518 times)

0 Members and 1 Guest are viewing this topic.

Offline turkey4me

  • Enthusiast
  • **
  • Posts: 237
  • The closer you are the less you can see!
Help, 2 & 3 won't fire
« on: August 26, 2010, 07:42:14 am »
My 77 750 F2 has been running great and suddenly cylinders 2&3 won't fire?  I have no spark at the plug.  Pulled the tank off to check the coil and all connections look good.  Put the tank back on and ta da I have fire.  Rolled the bike out of the garage and then lost fire to 2&3.  I can't seem to find the loose connection.  During this process my headlight bulb burnt out?  All fuses are good.  Bike starts just fine on 1&4 but the other half is asleep.    Hoping someone can walk me through what wires/connections to check with a multimeter?

The closer you are, the less you can see.

Offline Duke McDukiedook

  • Space Force 6 Star General
  • Really Old Timer ...
  • *******
  • Posts: 12,690
  • Wish? Did somebody say wish?
Re: Help, 2 & 3 won't fire
« Reply #1 on: August 26, 2010, 07:49:56 am »
Sounds like your tank maybe shorting out the coil to 2-3. Or you could be having condenser problems. Start at the ignition and work your way to the spark plugs looking for problems.
"Well, Mr. Carpetbagger. We got somethin' in this territory called the Missouri boat ride."   Josey Wales

"It's Baltimore, gentlemen. The gods will not save you." Ervin Burrell

CB750 K3 crat | (2) 1986 VFR750F

Offline BobbyR

  • Really Old Timer ...
  • *******
  • Posts: 12,367
  • Proud Owner of the Babe Thread & Dirty Old Man
Re: Help, 2 & 3 won't fire
« Reply #2 on: August 26, 2010, 10:25:55 am »
A few things to keep in mind which will help you.

1 and 4, 2 and 3 fire toghether so your problem is confined to one set of points, one coil and two ignition wires.

The engine block is part of the circuit so if one plug does not fire, the other will not, so you will run on two cyliders.

Snip a vrey small piece of the end of each plug wire so that the cap can screw into clean copper.

On my bike, it was the wire at the plug was the problem.
Dedicated to Sgt. Howard Bruckner 1950 - 1969. KIA LONG KHANH.

But we were boys, and boys will be boys, and so they will. To us, everything was dangerous, but what of that? Had we not been made to live forever?

Offline turkey4me

  • Enthusiast
  • **
  • Posts: 237
  • The closer you are the less you can see!
Re: Help, 2 & 3 won't fire
« Reply #3 on: August 27, 2010, 06:10:14 am »
I went through the tests below and had no issues.  Was able to get get spark at the 2&3 plugs using test 5 below.  But when I turn the starter over, no spark at plug.  Any ideas where to look next? 

1- Disconnect both the Yellow and Blue wire at the coil.

2- Check Black wire TO the coils with the key on.  Red probe on Black.  Black probe on a green wire or engine case where there is no paint.
If you don't have Battery potential (12V) or very close to it (do a compare to that at the battery terminals), the coils and ignition can't work.
Then fix the problem here.  Otherwise...

3- Reconnect the Black wires to the coils.  Now measure the disconnected Yellow and Blue wires each with the red meter probe and the Black probe on a green wire or engine case where there is no paint.
Again you should have Battery potential (12V) or very close to it.  If not, and you had voltage at the Black wire on the previous test, the coil(s) is bad.

4- Next lay #4 connected spark plug on the motor or wherever the spark plug body can touch a battery return path (engine, frame, Green wires, etc.)  With an extra wire, used as a temporary jumper, bridge the Blue wire to engine, frame, Green wires, etc.  Each time you remove the jumper, the spark plug should spark at the gap.

5- Next lay #3 connected spark plug on the motor or wherever the spark plug body can touch a battery return path (engine, frame, Green wires, etc.)  With an extra wire, used as a temporary jumper, bridge the Yellow wire to engine, frame, Green wires, etc.  Each time you remove the jumper, the spark plug should spark at the gap.


Offline 2wheels

  • Hot Shot
  • ***
  • Posts: 511
Re: Help, 2 & 3 won't fire
« Reply #4 on: August 27, 2010, 06:41:00 am »
You didn't mention checking the point gap.  Did you measure it yet?
1970 CB750 K0 (I can't believe I tossed my duck tail seat in the trash 30 years ago)

Offline Duke McDukiedook

  • Space Force 6 Star General
  • Really Old Timer ...
  • *******
  • Posts: 12,690
  • Wish? Did somebody say wish?
Re: Help, 2 & 3 won't fire
« Reply #5 on: August 27, 2010, 07:21:19 am »
Measure your dwell and check your coils with an ohmeter and compare to specs.
"Well, Mr. Carpetbagger. We got somethin' in this territory called the Missouri boat ride."   Josey Wales

"It's Baltimore, gentlemen. The gods will not save you." Ervin Burrell

CB750 K3 crat | (2) 1986 VFR750F

Offline turkey4me

  • Enthusiast
  • **
  • Posts: 237
  • The closer you are the less you can see!
Re: Help, 2 & 3 won't fire
« Reply #6 on: August 30, 2010, 05:37:02 am »
Found the problem. 
Ended up being a slightly loose points wire.  Tightened the nut one full turn and had fire!  While I was there decided to clean and dress all the connections just to be sure.

Thanks for the help and pointers.